Add Cloud Server (Rackspace, windows 2008 R2) to our physical domain. Domain has a Cisco 1841 router,

I need to add a new Rackspace (Windows Server 2008 R2) server to our current physical domain. I do not need the server to be a domain controller, just a persistant VPN connection between the two sites, which should then allow us to add the cloud server to our domain I have a Cisco 1841 router at the local site, and have access to it physically and admin rights to all local domain servers, and routers. I think the main issue is what VPN protocol needs to be used between the cloud server and physical domain, to get the server added as a computer on domain.